Self-Assembling Small Molecule Materials for Inexpensive Organic Solar Cells

APPLICATIONS OF TECHNOLOGY:

Solar cells
Light emitting diodes (LEDs)
Chemical biosensors
Memory devices
Thin films including thin film transistors
Radio frequency identification tags

ADVANTAGES:

Yields high performance organic PV cells
Uses small quantities of abundant, inexpensive materials
Is less toxic than other PV materials
No high temperature processing required

ABSTRACT:

Berkeley Lab researcher Jean Fréchet and colleagues have developed a class of materials that provides a new and controlled approach to producing high performance organic solar cells. These novel materials can be solution-processed into efficient devices due to their excellent self-assembling and film-forming properties.

The Berkeley Lab materials are created from small molecules with completely planar electron-rich moieties such as pyrene as the ends. The resulting conjugated small molecules are designed to self-assemble in an end-to-end fashion, mimicking a polymeric backbone (see figure). The end-groups of the molecules create strong and selective pi-pi interactions to enhance the self-assembly in a controlled manner. The resulting molecular entities mimic polymeric networks, with efficient charge transport properties, high fill-factors, and notable power conversion efficiencies in organic solar cells.

Organic polymers are widely used to produce organic solar cells and solar devices such as thin films and solar cell “fabrics.” PV devices from these polymers have many advantages over traditional silicon PV cells. Organic solar cells are flexible, lightweight, thin (requiring less material), inexpensive, and processable from solution at low temperatures (from room temperature to ~150°C). These materials are also less toxic than other thin-film PV materials, including CIGS (copper indium gallium selenide), GaAs (gallium arsenide), and CdTe (cadmium telluride).

However, conjugated polymers used to create these cells have drawbacks: the batches produced tend to vary in molecular weight, leading to potential inconsistencies in the resulting materials. Moreover, the material purification and synthesis processes are time intensive and tedious. Compared to polymers, small organic molecules, such as those developed by Berkeley Lab, are monodispersed and can be synthesized and purified with ease.

Figure: Attaching pyrene moieties to the ends of small molecules allows the molecules to self-assemble in a controlled, end-to-end fashion.